What are fuel cells?
Fuel cells are energy conversion devices that continuously transform the chemical energy of a fuel and an oxidant into electrical energy. This energy conversion process is accomplished by an electrochemical reaction whereby the reactants are consumed, by-product(s) are expelled, and heat may be released or consumed. Fuel cells will continue to generate electricity as long as both fuel and oxidant are available. Pure hydrogen, hydrocarbons, alcohols, and hydrazine are common fuels while pure oxygen and air are conventional oxidants.
Sir William Grove is widely attributed to be the discoverer of the fuel cell. From his experiments in 1839 on the electrolysis of water, Grove reasoned that it should be possible to reverse the process, reacting hydrogen with oxygen to generate electricity. Using fuel cells as electricity generation devices dates back only a few decades, when NASA began using fuel cells in the United States' space program. Fuel cells provided power for the Gemini and Apollo spacecraft. The space shuttle uses fuel cells to provide electricity and water.
How does a fuel cell work?
Fuel Cell Types
Polymer Electrolyte Membrane Fuel Cell
The Polymer Electrolyte Membrane Fuel Cell consists of a proton conducting membrane. This polymer membrane is very thin, 20-200 micrometers, flexible and transparent. The membrane is coated on both sides with Platinum (Pt) impregnated porous carbon electrodes. The membrane electrode assembly (MEA) is approximately 1mm thick. The operating temperature of the PEMFC is limited to 90 deg C or lower because the polymer membrane must be hydrated with liquid water to maintain adequate conductivity. Because of this low operating temperature the Platinum based catalyst is currently the only viable option.
The PEMFC has the highest power density with a range of 300-1000mW/cm2 and offers the most reliable fast start and on-off cycling. These characteristics make this fuel cell type highly suitable for transport applications and portable power although there are power generation applications currently available on the market.
The conversion efficiency of fuel bound energy to electricity of a PEMFC is 40 to 47% on a fuel (natural gas) LHV basis.
Molten Carbonate Fuel Cell
The Molten Carbonate Fuel Cell is a molten mixture of alkali (Na and K) carbonates Li2CO3 and K2CO3 retained in a ceramic matrix of LiOAlO2. The cell operates at temperature of 1100 to 1300 deg F or 600 to 700 deg C in order to keep the alkali carbonates in a highly conductive molten salt form, the carbonate ions providing ionic conduction. The electrodes are typically nickel based. Where the anode is a nickel/chromium alloy and the cathode is a lithiated nickel oxide. As CO2 is generated at the anode it is typically recycled to the cathode where it is consumed and since it is preheated by the combustion this improves the overall efficiency of the cell.
Because of the high operating temperature the MCFC can take a variety of fuel types such as methane, hydrogen, alcohols, and CO poisoning is nonexistent in fact the carbon monoxide acts as a fuel. The MCFC is best used in stationary applications like power generation and can achieve electrical efficiencies of up to 50%. In combined heat and power applications the efficiency goes up to 90%.
Solid Oxide Fuel Cell
The typical electrolyte is yttria-stabilized zirconia (YSZ) a solid, nonporous metal oxide, typically Y2O3 stabilized ZrO2 with the anode made from CoZrO2 or NiZrO2 cermet (cermet is a ceramic-metal mixture), while the cathode is made from strontium (Sr) doped lanthanum manganite (LaMnO3). The cell operates at 1200 to 1830 deg F or 650 to 1000 deg C and the mobile conductor is 02-.
Advantages of the SOFC are that there is no liquid electrolyte with its associated corrosion and electrolyte management problems, and with the high operating temperature internal reforming can be achieved. Overall thermal efficiencies are high; typically in the 45 to 50% range for conversion of the fuel (natural gas) bound energy to electricity on an LHV basis. Also, the exhaust heat from the SOFC can be recovered for the generation of steam for cogeneration purposes further increasing the efficiency. The efficiency for converting the fuel bound energy to electricity may be as high as >60% on an LHV basis. The high temperature of the SOFC, however, places stringent requirements on the materials of construction.
Phosphoric Acid Fuel Cell
The electrolyte consists of highly concentrated or pure, liquid phosphoric acid (H3PO4) saturated in a silicon carbide matrix (SiC); on either side of the electrolyte structure are catalysts made from platinum coated, porous, graphite electrodes. The operating temperature is maintained between 150 to 210 deg C, at lower temperatures, phosphoric acid tends to be a poor ionic conductor and at temperatures exceeding the maximum phosphoric acid undergoes an unfavorable phase change rendering it unsuitable as an electrolyte.
During operation the H3PO4 is evaporated to the environment and must therefore be replenished. Heat generated during cell operation is removed by either liquid or gas coolants which are routed through cooling channels in the cell stack. Electrical efficiencies are typically in the range of 40 to 47% with combined heat and power applications reaching into the 70% range.